Leicestershire 144 for 5 (Smith 65*) beat Derbyshire 143 (Durston 89, Ireland 5-22) by five wickets
Anthony Ireland starred in an extraordinary final over which involved a banned bowler and the fall of four wickets to set up Leicestershire's victory at Queen's Park.
Ireland's 5 for 22 beat his previous best figures of 4 for 11, also against Derbyshire - but he might never have bowled the over - or at least part of it.
Three of Ireland's wicket, plus a run out, came in the last over after Charlie Shreck had been ordered out of the attack for bowling his second full toss above waist height.
Derbyshire's dismal NatWest T20 Blast campaign continued when they lost to Leicestershire for the second time this season to remain rooted to the foot of the North Division.
Wes Durston blazed 89 from 63 balls, the fourth highest individual score for Derbyshire in the competition, but Marcus North was the only other batsman to reach double figures in a below-par total of 143.
Leicestershire lost their openers cheaply but Greg Smith settled any nerves with a unbeaten 65 from 56 balls to guide his team to their third victory of the season with four balls to spare.
The Foxes' decision to put the Falcons in on a white pitch was rewarded when Ireland struck twice in his first two overs, with Chesney Hughes losing his middle stump to an inside edge and Gareth Cross missing a drive.
Derbyshire's position would have been even worse if Ben Raine had not spilled a low catch when Durston clipped Shreck low to mid-wicket with 39 on the board, and he made them pay by driving and pulling his way to 50 off 33 balls.
Wayne Madsen was lbw reverse-sweeping Scott Styris, who combined with Jigar Naik to concede only 45 runs in eight overs despite Durston swinging a full toss from the New Zealander into the crowd behind the mid-wicket boundary.
Durston and North added 56 in nine overs, but Derbyshire could not break free and Naik held a low return catch to remove North before Durston's fine innings ended when he drove Raine to long-off.
That sparked the disintegration of the innings in bizarre fashion as Ireland took full advantage of Shreck's misfortune by taking three wickets in four balls as Derbyshire lost their last four in five for two runs, Mark Turner run out off the last ball.
It left Leicestershire a relatively easy chase on a small ground but they lost Niall O'Brien in the third over when he drove North low to short extra cover and after pulling Chesney Hughes for six, Josh Cobb top-edged a cut to Alex Hughes at cover.
At 39 for two, Derbyshire sensed an opening but Smith and Eckersley mixed placement with aggression to leave their side needing 69 from the last 10 overs.
The pair added 52 in seven overs before Eckersley needlessly skied a big drive at North to long-on, where Alex Hughes made no mistake.
Smith and Matthew Boyce took the visitors to within 24 of their target before Boyce swung Durston into the hands of deep mid-wicket.
But Smith completed a well-paced 50 from 48 balls and although Styris pulled the first ball of the final over to deep mid-wicket, it was too late for Derbyshire.
Ireland said: ``I didn't expect to be bowling the last over when Charlie Shreck started it but that's what can happen. It was a good performance and Greg has been in good form for us with the bat this year so it was nice to see him get us over the line at the end.''
Derbyshire skipper Madsen rued the calamitous end to his side's innings - as well as the last-over drama, they lost Durston to the last delivery of the 19th and Scott Elstone two balls before that.
Madsen said: ``I felt we were probably 10 or 15 short. Wes played brilliantly but we weren't able to get those extra runs at the back end. I thought we bowled really well in the first six overs but ultimately we were short, another 15 runs and we would have won today.''